Acute low-salinity stress induces gill injury, oxidative stress, and apoptosis via the JNK-JUN/MAPK axis in the swimming crab (Portunus trituberculatus)

Salinity fluctuation is a common environmental stressor threatening coastal crustaceans. Abrupt salinity drop can disrupt osmotic balance, trigger oxidative damage, and impair tissue homeostasis, yet the underlying molecular mechanism in Portunus trituberculatus remains insufficiently understood. In this study, we integrated histopathology, biochemical assays, transcriptomics, RNA interference (RNAi) and western blotting to explore the toxic effects and molecular responses of P. trituberculatus under acute low-salinity stress. The results revealed that the activities of antioxidant enzymes (SOD, CAT) and immune-related enzymes (ACP, AKP) in both anterior and posterior gills presented a notable trend of rising first and then falling. Throughout the experiment, the activity of Na⁺/K⁺-ATPase was consistently higher in posterior gills, indicating their dominant role in osmoregulation. In addition, acute low-salinity exposure triggered detachment of some epithelial cells and formation of large vacuoles, along with a marked increase in cell apoptosis in posterior gills. Transcriptomic analysis demonstrated that the JNK-JUN/MAPK axis was specifically activated in posterior gills to mediate stress responses. Western blotting verified that RNAi-mediated JNK knockdown significantly down-regulated the expression of JNK and JUN. Moreover, JNK inhibition exacerbated epithelial detachment, vacuolization and apoptosis in posterior gill tissues, further confirming the regulatory function of the JNK-JUN/MAPK axis in stress adaptation. Collectively, these findings uncover the core mechanisms by which the JNK-JUN/MAPK axis modulates gill injury, apoptosis and antioxidant responses upon acute low-salinity stress and provide potential molecular biomarkers for environmental risk assessment of coastal crustaceans.